(define (solve n k vx va)
  (- (let loop ([r (+ (vector-ref vx (- k 1)) (vector-ref va (- k 1)))]
                [i k])
       (cond [(= i n) (- n 1)]
             [(< r (vector-ref vx i)) (- i 1)]
             [else (loop (max r (+ (vector-ref vx i) (vector-ref va i))) (+ i 1))]))
     (let loop ([r (- (vector-ref vx (- k 1)) (vector-ref va (- k 1)))]
                [i (- k 2)])
       (cond [(= i -1) 0]
             [(> r (vector-ref vx i)) (+ i 1)]
             [else (loop (min r (- (vector-ref vx i) (vector-ref va i))) (- i 1))]))
     -1))

(define (main args)
  (let* ([n (read)]
         [k (read)]
         [vx (vector-tabulate n (^_ (read)))]
         [va (vector-tabulate n (^_ (read)))])
    (print (solve n k vx va)))
  0)